Tutorial on the Design and Characterization of Class-B and AB Amplifiers

In this episode Shahriar continues his investigation of discrete Bipolar amplifier design. The advantages and disadvantages of Class-A amplifiers are explained. The conceptual schematic of a Class-B amplifier is presented which leads to the introduction of Class-AB amplifier circuit to overcome the ‘dead-zone’ impairment of a push-pull Class-B design. To further improve the Class-AB amplifier and lower its input impedance, a final Class-A followed by Class-AB amplifier is presented The component parameters are calculated and the schematics is explained in detail. All circuits are then implemented on a breadboard and tested both in the time domain and frequency domain. All schematics can be downloaded from here.

Thank you for the good video.
At the minute 28:15 you have talked about the X Y relation ship, what I understand from you that the horizontal (X) is the input and vertical (Y) is the output, but this is reversed, the horizontal (X) is the output and vertical (Y) is the input.

Shahriar, this was a great video (as always). As you have a knack for explaining things and a clear understanding of RF (which other video bloggers do not, sorry EEVBlog), I was wondering if I could convince you to expand the discussion of amplifier circuits in the VHF or UHF realms. I’ve been reading and learning about this stuff for a long time, but it never seems to click. Especially in regards to how noise figure works and can be minimized.

Hi Shahriar: looking at the spectrum display at 31:04 it shows the 50KHz marker at -1.90dBm. Why is that, why doesn’t the fundamental start at 0dBm? Should you adjust the analyzer vertical to start the fundamental at 0dBm so that you can measure the height of the harmonics (if you were going to calculate the distortion for example)?

The pdf version has the correct diode orientation but is still missing bias resistors. If you’d build the circuit this way, it would not work. The following circuit diagram shows a correct AB amplifier circuit. (I just entered “AB amplifier” in google image search, it was one of the first hits.)

It was quite thrilling for me to watch the video with the incorrect AB-Amplifier schematic in the image for such a long time. When Shahriar explained the voltages in the AB-Amplifier circuit I was so sure he would recognize and correct the error any second.. (I watch the videos on my cell phone and therefore had no access to the comments or the notes in the youtube video.)

Besides this little error this is an excellent video. Thanks to Shahriar for taking the time to make this videos!

The distortion is due to “clipping”, that is, the output cannot reach what is required.
For the “top-half” of the circuit, if we consider a 2.5V input (5Vpp) you would need a 3.1 positive rail (Vm+Vbe=2.5+0.6=3.1V) to provide the voltage required on Q3’s base. Since Q3 needs its base to be Vbe above Ve to conduct. And likewise (but in the opposite direction) for the lower half of the circuit (Q2, Q4).
To overcome this increase the positive rail and decrease the negative rail. This increase should be at least one Vbe above the maximum input swing for the positive rail and one Vbe below the maximum negative swing for the negative rail. To be safe the supply should be more one Vbe over (say 1V instead – depends on the transistors, etc…).